Grounding 1908
- Thread starter bennie
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Re: Grounding 1908
After reading the article in the NEC digest, thanks to Karl, I don't agree with the pictoral illustration. It doesn't make sense.
There seems to be some confusing dates on the documents I have examined so far. I think 1914 may be the date when secondary grounding became mandatory.
Do I have any agreement that grounding for lightning or transient surges was not the original reason for grounding (earthing)?
[ June 22, 2003, 12:33 PM: Message edited by: bennie ]
After reading the article in the NEC digest, thanks to Karl, I don't agree with the pictoral illustration. It doesn't make sense.
There seems to be some confusing dates on the documents I have examined so far. I think 1914 may be the date when secondary grounding became mandatory.
Do I have any agreement that grounding for lightning or transient surges was not the original reason for grounding (earthing)?
[ June 22, 2003, 12:33 PM: Message edited by: bennie ]
Ed MacLaren
Senior Member
Re: Grounding 1908
Keep in mind that delta connected secondarys were not required to be grounded, while those connections that had a neutral conductor (Y connected three-phase and 3-wire edison) were.
I believe that reinforces my argument (my post above) that the original purpose was to limit the voltage to ground under normal (no-fault) conditions.
Ed
[ June 22, 2003, 01:05 PM: Message edited by: Ed MacLaren ]
I agree with you.
Keep in mind that delta connected secondarys were not required to be grounded, while those connections that had a neutral conductor (Y connected three-phase and 3-wire edison) were.
I believe that reinforces my argument (my post above) that the original purpose was to limit the voltage to ground under normal (no-fault) conditions.
Ed
[ June 22, 2003, 01:05 PM: Message edited by: Ed MacLaren ]
Re: Grounding 1908
Thanks Ed, for your acknowledgement. I agree there was originally only one reason for connecting to the dirt. It is as you stated.
A fault through the transformer will be cleared by the common neutral conductor. Ground contact is not necessary.
I have a problem understanding the role of an earth connection in reducing a high voltage due to crossing wires. The high voltage will feed into the premises through the line conductors, and probably burn through the neutral at the point of contact.
If there is only one reason, for grounding, then how does 25 ohms have any impact? Is the 25 ohms being mis-understood as to its application? Is it actually 25 ohms maximum from one electrode to the other?
Does the 660 watts maximum, allowed on each side of a Edison circuit, in the 1890s, figure in the equation?.
A 6 amp fuse would blow on a fault with a path resistance of 25 ohms.
I am only speculating, maybe something will make sense.
Thanks Ed, for your acknowledgement. I agree there was originally only one reason for connecting to the dirt. It is as you stated.
A fault through the transformer will be cleared by the common neutral conductor. Ground contact is not necessary.
I have a problem understanding the role of an earth connection in reducing a high voltage due to crossing wires. The high voltage will feed into the premises through the line conductors, and probably burn through the neutral at the point of contact.
If there is only one reason, for grounding, then how does 25 ohms have any impact? Is the 25 ohms being mis-understood as to its application? Is it actually 25 ohms maximum from one electrode to the other?
Does the 660 watts maximum, allowed on each side of a Edison circuit, in the 1890s, figure in the equation?.
A 6 amp fuse would blow on a fault with a path resistance of 25 ohms.
I am only speculating, maybe something will make sense.
bphgravity
Senior Member
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Re: Grounding 1908
This is extracted from the "Library of Practical Electricty, Central Stations", by Terrell Croft - 1917 Edition.
"....Where the neutral is grounded to ensure the automatic disconnection of a feeder in the event of a ground on it, a resistance may be inserted between the neutral point of the generator or transformer and ground, to prevent the flow of excessive current in case of an accidental ground on the line. Such a resistance should be proportional that it would permit enough current to flow to operate the automatic oil switches, but would at the same time prevent the flow of a dangerously large current....."
This seems to imply at the time that the use of Earth grounding was indeed used for clearing of faults, however the resistance of the fault path needed to be calculated. It appears that at the time it was only desirable to allow enough current to flow to trip the OCD, but not more than necessary for that purpose. Is it possible that 25-ohms became the industry standard. or average that would allow utility overcurrent protection to operate while at the same time not placing large currents to ground?
I will keep searching! The answer is out there!
This is extracted from the "Library of Practical Electricty, Central Stations", by Terrell Croft - 1917 Edition.
"....Where the neutral is grounded to ensure the automatic disconnection of a feeder in the event of a ground on it, a resistance may be inserted between the neutral point of the generator or transformer and ground, to prevent the flow of excessive current in case of an accidental ground on the line. Such a resistance should be proportional that it would permit enough current to flow to operate the automatic oil switches, but would at the same time prevent the flow of a dangerously large current....."
This seems to imply at the time that the use of Earth grounding was indeed used for clearing of faults, however the resistance of the fault path needed to be calculated. It appears that at the time it was only desirable to allow enough current to flow to trip the OCD, but not more than necessary for that purpose. Is it possible that 25-ohms became the industry standard. or average that would allow utility overcurrent protection to operate while at the same time not placing large currents to ground?
I will keep searching! The answer is out there!
Ed MacLaren
Senior Member
Re: Grounding 1908
An elderly gentleman (older than us ) who was both a contractor, and later our chief inspector, recently gave me his collection of old electrical books. Most of them were published in the twenties and thirties, and make very interesting reading.
Bennie, you commented - "I have a problem understanding the role of an earth connection in reducing a high voltage due to crossing wires."
Here is an excerpt from a 1947 NEC Handbook -
"Suppose that a conductor of a grounded high-voltage system makes accidental contact with one of the wires of the secondary distribution mains or with one of the service wires A. Such a connection is indicated by the dotted line in the sketch.
FIG 1 - An interior wiring system with a system ground at the service entrance.
Conductor A is connected to conductors
B and C through the transformer windings, also through whatever load (lamps and motors) may be in operation at the time. If, when the accidental cross occurs, there is no ground connection to any conductor of the secondary system, the voltage above ground of secondary conductor A will be the same as that of the high-tension wire
with which it is in contact, and the voltage to ground of the other conductors B and C will be nearly the same, Under these conditions, a person touching any one of the secondary conductors and being at the same time in contact with a grounded object will receive a shock due to the difference of potential of approximately 2,300 volts.
All conducting parts of the wiring system and all lighting, power, and heating apparatus connected to the system are insulated for a normal voltage to ground of not over 600 volts. Certain parts of the system may be insulated for a normal voltage not exceeding 250 volts. If the voltage to ground of the entire system and all apparatus connected to it is increased to 2,300 volts, it is very probable that the insulation will break down in numerous places. From every point where such a breakdown occurs a current will flow to ground, and anyone of these stray currents may heat some conducting material through which it flows to such a high temperature that it will ignite any woodwork or other combustible material with which the heated conductor is in contact.
Let us now consider what will take place when the neutral conductor of the secondary is well grounded and an accidental cross with one of the high-tension wires occurs. A current will flow from the high-tension wire to secondary wire A, through the transformer winding to secondary wire
B, thence to ground, and through the ground back to the grounded neutral of the primary. The amount of this current will depend upon the voltage maintained by the transformer supplying the primary line and upon the total impedance of the circuit and is, of course, entirely indeterminate.
Under these conditions the voltage above ground of the interior wiring system will be the voltage drop in the grounding conductor and in the contact between the electrode and the ground. A current of 100 amps and a resistance of 2 ohms from the neutral conductor to ground would
represent rather severe conditions. The voltage drop in such a case would be I X R or 100 X 2 = 200 volts; thus the normal potential difference between any part of the wiring system and ground would be increased by only 200 volts.
The resistance of a good ground to a water piping system will commonly be much less than 2 ohms, with a corresponding reduction in the possible voltage to ground when a heavy current flows through the grounding connections. The effective grounding of the secondary system, by thus preventing an excessive increase of the voltage to ground, eliminates to a very great extent both the hazard to life and the fire hazard in case of an accidental cross between the secondary system and a high-tension conductor."
Ed
An elderly gentleman (older than us
) who was both a contractor, and later our chief inspector, recently gave me his collection of old electrical books. Most of them were published in the twenties and thirties, and make very interesting reading.Bennie, you commented - "I have a problem understanding the role of an earth connection in reducing a high voltage due to crossing wires."
Here is an excerpt from a 1947 NEC Handbook -
"Suppose that a conductor of a grounded high-voltage system makes accidental contact with one of the wires of the secondary distribution mains or with one of the service wires A. Such a connection is indicated by the dotted line in the sketch.
FIG 1 - An interior wiring system with a system ground at the service entrance.
Conductor A is connected to conductors
B and C through the transformer windings, also through whatever load (lamps and motors) may be in operation at the time. If, when the accidental cross occurs, there is no ground connection to any conductor of the secondary system, the voltage above ground of secondary conductor A will be the same as that of the high-tension wire
with which it is in contact, and the voltage to ground of the other conductors B and C will be nearly the same, Under these conditions, a person touching any one of the secondary conductors and being at the same time in contact with a grounded object will receive a shock due to the difference of potential of approximately 2,300 volts.
All conducting parts of the wiring system and all lighting, power, and heating apparatus connected to the system are insulated for a normal voltage to ground of not over 600 volts. Certain parts of the system may be insulated for a normal voltage not exceeding 250 volts. If the voltage to ground of the entire system and all apparatus connected to it is increased to 2,300 volts, it is very probable that the insulation will break down in numerous places. From every point where such a breakdown occurs a current will flow to ground, and anyone of these stray currents may heat some conducting material through which it flows to such a high temperature that it will ignite any woodwork or other combustible material with which the heated conductor is in contact.
Let us now consider what will take place when the neutral conductor of the secondary is well grounded and an accidental cross with one of the high-tension wires occurs. A current will flow from the high-tension wire to secondary wire A, through the transformer winding to secondary wire
B, thence to ground, and through the ground back to the grounded neutral of the primary. The amount of this current will depend upon the voltage maintained by the transformer supplying the primary line and upon the total impedance of the circuit and is, of course, entirely indeterminate.
Under these conditions the voltage above ground of the interior wiring system will be the voltage drop in the grounding conductor and in the contact between the electrode and the ground. A current of 100 amps and a resistance of 2 ohms from the neutral conductor to ground would
represent rather severe conditions. The voltage drop in such a case would be I X R or 100 X 2 = 200 volts; thus the normal potential difference between any part of the wiring system and ground would be increased by only 200 volts.
The resistance of a good ground to a water piping system will commonly be much less than 2 ohms, with a corresponding reduction in the possible voltage to ground when a heavy current flows through the grounding connections. The effective grounding of the secondary system, by thus preventing an excessive increase of the voltage to ground, eliminates to a very great extent both the hazard to life and the fire hazard in case of an accidental cross between the secondary system and a high-tension conductor."
Ed
Re: Grounding 1908
Excellent articles. I have no problem understanding them, it makes good sense.
I note the absence of any reference to lightning or transient issues.
It is interesting to read the ground electrode system will not clear a low side fault, but will clear a high side.
One important bond left out of the schematic, is from the primary system neutral to the secondary neutral. A high voltage would use that ground if the premises did not have a ground electrode system.
I am now searching for the date of the first lightning arrestor or MOV, for power distribution systems.
Excellent articles. I have no problem understanding them, it makes good sense.
I note the absence of any reference to lightning or transient issues.
It is interesting to read the ground electrode system will not clear a low side fault, but will clear a high side.
One important bond left out of the schematic, is from the primary system neutral to the secondary neutral. A high voltage would use that ground if the premises did not have a ground electrode system.
I am now searching for the date of the first lightning arrestor or MOV, for power distribution systems.
Re: Grounding 1908
I will try a revive this topic...The reason I started this thread was to trace the origin of grounding as applied to the separately derived system.
From 1908 Engineering Text Book...On large three wire systems the neutral is best to be grounded. On small or isolated systems, it is best to leave the neutral ungrounded.
Small systems and isolated systems were not grounded until about 1914.
I will try a revive this topic...The reason I started this thread was to trace the origin of grounding as applied to the separately derived system.
From 1908 Engineering Text Book...On large three wire systems the neutral is best to be grounded. On small or isolated systems, it is best to leave the neutral ungrounded.
Small systems and isolated systems were not grounded until about 1914.
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